Known processes heretofore in use for producing a synthetic quartz glass for, e.g., an optical member such as a mirror or lens include a process which comprises forming a porous quartz glass base by a vapor-phase reaction method and heating this base to undergo vitrification (see, for example, patent document 1).
In the process for synthetic quartz glass production disclosed in patent document 1, a porous quartz glass base of a substantially cylindrical shape is formed by the so-called VAD (vapor-phase axial deposition) method in which a silicon compound, e.g., silicon tetrachloride, is introduced into an oxyhydrogen flame to synthesize fine quartz glass particles through flame hydrolysis and the fine quartz glass particles are deposited on a rotating substrate. In the process, this base is then heated to a temperature not lower than the vitrification temperature to obtain a synthetic quartz glass.
The synthetic quartz glass thus obtained contains hydroxyl groups (OH groups) typically in an amount of about 100 to 300 ppm (on a weight basis; the same applies hereinafter) because of the nature of the process. It is known that these OH groups reduce the transparency to infrared, transparency to vacuum ultraviolet, and other properties of the synthetic quartz glass. Various techniques for reducing the amount of OH groups in a synthetic quartz glass have been proposed (see, for example, patent document 2).
In the process for synthetic quartz glass production disclosed in patent document 2, a porous quartz glass base is held, before being heated to a temperature not lower than the vitrification temperature, for a certain time period in a fluorine-containing atmosphere having a pressure of from 100 Pa to 101 kPa and a temperature of from room temperature to 800° C. to thereby replace OH groups contained in the porous quartz glass base with fluorine atoms, whereby a reduction in OH group amount is attained.
Patent Document 1: JP-A-62-72536
Patent Document 2: JP-A-2003-201124
In general, the synthetic quartz glass obtained via a porous quartz glass base of a substantially cylindrical shape formed by the VAD method described above has a convex type OH group concentration distribution curve which indicates that the OH group concentration around the core is relatively high. Such OH group concentration distribution is a factor which causes the synthetic quartz glass to fluctuate in refractive index (striae). It is therefore preferred that a synthetic quartz glass for an optical member or the like should have a uniform OH group concentration.
According to the process for synthetic quartz glass production disclosed in patent document 2, it may be possible to narrow the fluctuation range of the OH group concentration by reducing the absolute amount of OH groups contained in the synthetic quartz glass. However, there is a desire for a more positive measure in attaining a uniform OH group concentration.